/Engine/Source/Runtime/OpenGLDrv/Public/OpenGLES.h
On iOS both glMapBufferOES() and glBufferSubData() for immediate vertex and index data is the slow path (they both hit GPU sync and data cache flush in driver according to profiling in driver symbols). Turning this to false reverts back to not using vertex and index buffers for glDrawArrays() and glDrawElements() on dynamic data.
static bool SupportsFastBufferData()